The main industrial process that involves use of a cellulose solution is the manufacture of shaped articles, such as films and fibers. The viscose process has been the principal method for the manufacture of regenerated cellulose fibers in the past. The viscose process consists of steeping cellulose sheets in sodium hydroxide, shredding the sheets, aging the shredded sheets to form alkali cellulose, treating the alkali cellulose with carbon disulfide to form a cellulose xanthate solution, ripening and filtering the solution, extruding the solution through the small holes of a spinneret into an acid coagulating bath to form filaments, stretching the filaments and then washing, drying and packaging the fibers.
Even at present, the principal cellulose dissolving method that is used on an industrial scale for the purpose of forming fibers is the viscose method and the regeneration of cellulose fibers from cuprammonium cellulose solution is also used to some extent. Both of these processes involve the formation of a cellulose derivative. Films and fibers can be formed from these cellulose derivatives via regeneration of the solution of the derivative into acidic aqueous medium. These processes, however, are costly due to the use of numerous steps involved in preparing satisfactory solutions for fiber or film regeneration, which steps require expensive equipment or require high temperatures or high pressures or both. Also, in the viscose process, carbon disulfide is required in the preparation of the cellulose xanthate solution. The carbon disulfide is not part of the final regenerated cellulose and its presence in the system contributes to environmental problems.
There are other known cellulose-dissolving methods using a metal complex, although these methods have not generally been utilized industrially. As the metal complex used for dissolving cellulose, Cadoxene (cadmium/ethylenediamine/alkali), Coxene (cobalt/ethylenediamine/alkali), Zincoxene (zinc/ethylenediamine/alkali), Nioxene (nickel/ethylenediamine/alkali) and EWNN (iron/tartaric acid/alkali) have been used. Methods utilizing a metal complex, however, are not superior to the viscose method or the cuprammonium method, because toxic components, such as heavy metals and amines, are used and these methods are disadvantageous from an economical viewpoint.
In order to overcome the defects of the known cellulose-dissolving methods, a method for dissolving cellulose directly in an organic solvent or inorganic solution has been sought for a substantial period of time. As a result, various methods have been developed and proposed. Most of these methods, however, use a complicated multi-component solvent system which is expensive, require high temperatures or high pressures or both, and none of these methods have yet been used in practice, primarily because of toxicity, explosiveness and difficulty in recovering the solvents. Moreover, these methods are similar to the viscose method or the cuprammonium method in that the cellulose is converted to a derivative which is then dissolved. For example, U.S. Pat. No. 4,097,666 to Johnson, et al. describes a method of using a solvent system consisting of dimethyl sulfoxide and paraformaldehyde to dissolve cellulose in the form of methylol cellulose. It is also known that it is possible to dissolve cellulose in the form of cellulose nitrate utilizing dimethylformamide and N.sub.2 O.sub.4. Solvent systems utilizing SO.sub.2 /amine and dimethylformamide/chloral have also been used to dissolve cellulose in the form of a derivative.
It is also known that if the degree of polymerization, hereinafter referred to as "DP" of cellulose is below about 20, the cellulose is soluble in an aqueous alkali solution and in hot dimethyl sulfoxide. However, cellulose fibers and films formed from such cellulose solutions do not have satisfactory properties and such cellulose solutions do not have a practical use for forming fibers and films. Apparently, the reason why low DP cellulose is soluble in an aqueous alkali solution is that the polymeric characteristics of cellulose, such as the molecular configuration defined by the hydrogen bond, is lost.
It is also known that an aqueous solution containing about 10 percent by weight of sodium hydroxide shows a strong swelling action for cellulose having a high degree of polymerization. It has been reported in the Journal of Prakt. Chem N.F. 158, 233 (1941) that natural cellulose, mercerized cellulose and regenerated cellulose are soluble in an aqueous 10 percent by weight solution of sodium hydroxide. Although the dissolving conditions and polymer concentrations are not described, it was reported that natural cellulose and mercerized cellulose are soluble for cellulose having a DP up to 400 and that regenerated cellulose is soluble if the DP is up to 1200. However, the text of this reference indicates that even a highly swollen gel is considered to be in the category of a "dissolved polymer".
U.S. Pat. No. 4,634,470 to Kamide, et al. reports substantial experiments based upon teachings provided by the Journal of Prakt. Chem. article. The Kamide, et al. patent indicates that it was found that the solubility of cellulose in an aqueous 10 percent by weight sodium hydroxide solution at -5.degree. C. to 5.degree. C. is influenced by the polymer concentration and the degree of polymerization. For example, it is reported that cellulose 5 dispersions, even at a cellulose concentration of 0.5 percent by weight in 10% sodium hydroxide, when subjected to centrifugal separation at 20,000 rpm for 46 minutes, proved that the cellulose is not completely dissolved. Accordingly, it is concluded in the Kamide, et al. patent that the term "soluble" used in the Journal of Prakt. Chem means that cellulose is dissolved at a very low concentration in the gel-containing state. Such a solution of a low concentration of cellulose is of no practical value. The Kamide, et al. patent indicates that low solubility of cellulose in sodium hydroxide is confirmed from the fact that an aqueous alkali solution was used for the fractional dissolution of celluloses. This fractional dissolution is an operation of separating cellulose into an alkali soluble portion and an alkali/insoluble portion, according to the molecular weight of the cellulose and the aggregation state of the molecular chains, and that the soluble portion includes a gel. These facts indicate that it is technically very difficult to dissolve substantially all of a cellulose having a high degree of polymerization at a high cellulose concentration in an aqueous alkali solution having a single alkali composition. A cellulose/alkali solution has not been utilized as a solution in the formation of fibers or films in the cellulose industry.
The Kamide, et al. patent is directed to a method for preparing a cellulose solution by maintaining a mixture comprising 100 parts of cellulose which may have any crystal form and 10 to 100 parts by weight of a hydrogen bond-cleaving solution at 100.degree. C. to 350.degree. C. under a pressure of 10 to 250 atmospheres for a period of time and promptly exposing the mixture to explosive decompression. The cellulose mixture is then mixed with an aqueous alkali solution maintained at a temperature not higher than 50.degree. C. The cellulose mixture is stirred at a temperature not higher than 10.degree. C. to dissolve the cellulose in the aqueous alkali solution. The hydrogen bond-cleaving agent can be water, an aqueous solution of an alkali, an aqueous solution of an inorganic acid, an aqueous solution of a salt, an aqueous solution of hydrogen peroxide, an aqueous solution of a urea compound, an aqueous solution or dispersion of an amphoteric oxide or amphoteric hydroxide, a glycol, an amide, a sulfoxide, a halogenated acetic acid and a polyphenol.
U.S. Pat. No. 4,302,252 to Turbak, et al. is directed to a method for dissolving cellulose wherein cellulose is activated by penetration of the cellulose with a polar medium and mixing at a temperature at which no significant degradation occurs with an amide selected from the group consisting of dimethylacetamide, 1-methyl-2-pyrrolidinone and mixtures thereof and from 3 to 15 percent by weight of lithium chloride. The cellulose is then dissolved in the amide and lithium chloride in the substantial absence of any polar medium other than the amide. It does not appear that the process taught in Turbak, et al. has been reduced to practice by industry since the issuance of the patent. This appears to be due to the expense associated with the lithium chloride required to practice the invention which makes the practice of the invention economically prohibitive.
From the foregoing discussion of the prior art, it can be seen that it is technically extremely difficult to dissolve substantially all of a cellulose having a high DP, I0 which includes any of the native forms of cellulose or celluloses isolated by industrial processes, at a high cellulose concentration in an aqueous alkali solution having a single alkali composition.